From https://ogrforum.ogaugerr.com/...bowser-brill-trolley . Leaving this here for future reference...
Front row L-R: Bowser ST Birney, Western Hobbycraft St. Louis, Bowser Brill, IR/Atlas Brill.
Back row L-R: Bachmann/Williams Peter Witt, MTH PCC, MTH Brill.
As you can see, both the Bowser Brill and MTH Brill are larger than the IR Brill, which is arguably closer to traditional/S scale.
The Bowser Birney is scaled to match the Bowser Brill. The Lionel Birney and MTH single truck car have similar size and proportions.
The Western Hobbycraft St. Louis car is similar to the Bowser Brill and MTH PCC, sizewise.
Now, let's go for a ride on Single Truck Street! 
L-R: General Models Birney, converted from vibrating motor to rail speeder truck:
Corgi Birney, likewise:
Bowser Birney:
Lionel Birney, converted to 2-rail Wagner truck and working on overhead:
Stock Lionel bump and go Birney:
MTH Bump and Go Brill.
Lionel Birney and MTH Brill compared.
Bowser Birney and Brill compared. H'm. Autofocus off on that one.
Anyhoo, that should give you a feel for relative sizes.
Hope this helped! 
Mitch
